1. Field of the Invention
The present invention relates to a novel heat sensitive recording material. More particularly, it relates to a heat sensitive recording material with enhanced color forming and image stability.
2. Description of the Prior Art
Heat sensitive recording materials having a heat sensitive layer which forms color by heating and is coated on a support, such as paper, synthetic paper and plastic film, are widely used in thermal printers of facsimiles, portable calculators, microcomputers and the like, in thermo-pen recorders of cardiograph and analytical instruments, in train tickets, in labels for POS at super markets and the like applications.
The heat sensitive recording materials are generally produced by the following processes: a color former of a colorless or light color leuco dyestuff, such as lactone dyestuffs, lactam dyestuffs and spiropyran dyestuffs, and a developer which develops color by reaction with the color former by heating are dispersed in a disperse medium separately after each of the components have been pulverized with a ball mill or a sand mill; these dispersions are mixed with addition of a binder to form a single dispersion; a coating dispersion is prepared from this dispersion, when necessary, with addition of a wax, a sensitizer, a surface active agent, a defoaming agent, an inorganic pigment and the like; and the coating dispersion is coated on a support such as paper and dried to form the heat sensitive color forming layer.
As a developer having good image stability, 2,4'-dihydroxydiphenylsulfone and 4,4'-dihydroxydiphenylsulfone are used in Laid Open Japanese Patent Applications 1981-127486 and 1988-3991. However, these compounds show inferior heat response and satisfactory concentration of the color forming cannot be obtained. To overcome the problem, a sensitizer is generally added to the developer to decrease the temperature required for the color developing. As the sensitizer for this purpose, paraffin wax, amides such as fatty acid amides, esters such as dimethyl terephthalate and ethers are generally used. However, these sensitizers do not have sufficient ability with respect to the concentration and the sensitivity of the color forming and are not satisfactory because of the problem that in, age stability is inferior even when an image of high concentration of the color forming is obtained and the image deteriorate with the passage of time.